Wind turbine blade

ABSTRACT

The invention relates to a wind turbine blade comprising a number of pre-fabricated strips arranged in a sequence along the outer periphery. The strips consist of a fibrous composite material, preferably carbon fibres, and consist of a wooden material, preferably plywood or wooden fibres held in a cured resin. The advantage is that it is possible to manufacture blades for wind turbines which are very stiff and generally have a high strength, but which nevertheless are easy to manufacture and also is much cheaper to manufacture compared to conventional manufacturing techniques. The invention also relates to methods for manufacturing prefabricated strips and for manufacturing wind turbine blades.

The present invention relates to a blade for wind turbines, in whichblade the periphery layer of the cross section of the blade

BACKGROUND OF THE INVENTION

Wind turbine blades are today manufactured comprising a bearing centralinner beam, commonly of a hollow, square cross-section and made from aglass fibre and resin composite, surrounded with two shells forming theupper and the lower outer surface of the blade and determining theaerodynamic properties thereof.

The shells may be of a single layer or at least along a part of thecircumference be a sandwich construction comprising two parallel layersof glass fibres and resin having a space in between filled with e.g. apolyurethane foam. The use of a wooden material to reinforce the innerside of a single layer shell or to fill the space of a sandwichconstruction is well known.

It is realised that the forces and torque increase steeply with theincreasing length of blades and that the strength and stiffness of theinner beam must be steeply increased as well for the known blades, asthe shells only contribute minor to the overall load bearing propertiesof the blade.

In order for the shell to bear a substantial part of the forces of theinner beam, the above discussed structures that are reinforced withwooden material require for larger dimensions of blades a thickness ofthe wooden layer that would increase the weight of the bladesignificantly, thus causing increased stresses to the blade.

It is the object of the invention to provide a wind turbine blade havingproperties of laminated products, i.e. high strength in comparison withthe amount of material and lower production cotsts compared to solidproducts, but where the strength compared to the costs of manufacturingthe blade is highly increased compared to prior art blades.

DESCRIPTION OF THE INVENTION

This object is obtained by a blade, which blade over a substantiallongitudinal part comprises a layer along an outer periphery of thecross-section of the blade a blade, said layer constituted by aplurality of pre-fabricated strips arranged in a sequence along theouter periphery of the blade

By the term “a substantial longitudinal part” is understood a partextending over at least a third of the total length of the blade fromtip to hub, preferably over at least half of the total length of theblade. According to a preferred embodiment, 60–85% of the total length,such as about 70%, comprises such layer.

Thereby, the optimal material properties may be obtained by combiningdifferent types of strips, such as pultruded fibrous composite stripscomprising different fibres, such as carbon fibres, glass fibres and/ornatural fibres, wooden strips, composite strips formed as hollow tubesetc. Each of the types of strips are much simpler, and thus cheaper, tomanufacture than to form a whole blade, and the strips may be joint bysuitable methods, such as by injection of resin or by vacuum infusion ofresin.

According to the invention, a wind turbine blade may be obtained, whichreduces the forces and torque on the inner beam. Furthermore, theresistance against tension and compression forces in a layer near theouter periphery of the shell provides the blade with an improvedstructural efficiency with respect to an edge-wise bending mode.

Accordingly, in a preferred embodiment, at least some of thepre-fabricated strips are made from pultruded fibrous compositematerial, such as carbon-resin.

Thereby, a construction is obtained with an excellent stiffness, butwhich is not prone to buckling. Thus, the inner structure of the blademay be made of a lighter construction, e.g. by replacing the commonlyused inner beam of a square cross section with two lighter webs at theleading edge and the travelling edge, respectively.

The periphery layer may, in a preferred embodiment, be assembled byinjection of resin or by vacuum infusion of resin. Use of resin infusionleads to a speedy, healthy and safe manufacturing process, leaving no oronly very few voids in the resin. A limitation of the number of voidsreduces subsequent finishing. A very little amount of the fibres in theblade is actually infused. The resin is mainly a glue rather than amatrix. This results in a structure being more tolerant to any possiblevoids.

According to one, preferred embodiment, the blade over a substantiallongitudinal part comprises a layer along the outer periphery of itscross-section, wherein the layer at least partly is constituted bystrips of a wooden material and strips of a fibrous composite materialin an alternating sequence along the outer periphery.

Thereby, the excellent stiffness of fibrous composite materials and thehigh resistance against bulking of wooden materials is combined toachieve a shell with suitable properties in a cost-efficient manner.

An especially advantageous embodiment comprises at least some stripsmade from a wooden material, preferably plywood used as the woodenmaterial, and natural fibre pultrusions, preferably carbon fibrepultrusions, as the fibrous composite material.

The advantages obtained by this embodiment are that the materials arecompatible and both are having low thermal expansion coefficients. Bothtypes of material work at similar low range of strains resulting in thepossibility of stiffer blades compared to the weight of the blades.Also, natural fibres may be prone to buckling, and although wood isbulky, wood is not prone to buckling, thus also for this reason, the twotypes of material are very complementary.

The strips may in general be made from wood, laminated wood, pultrusionsfrom any fibre man-made or natural with any resin, thermoset,thermoplastic, man-made or naturally derived, foam plastics, lightweightcore materials in any proportion. At least some of the pre-fabricatedstrips are advantageously formed from a fibrous composite material. Thefibres of the fibrous material may be any known fibre having suitableproperties to reinforce the wood composite, such as carbon fibres, glassfibres, Kevlar fibres, natural fibres, e.g. from hemp or flax, coirfibres, etc. or any combination thereof.

As example, carbon has a higher strain to failure than wood. Carbon actsas stiffening additive but wood fails first. This has been takenadvantage of in coupon testing to prove strength of carbon and woodseparately. Adding carbon and thus the possibility of using thinnerskins nay reduce skin buckling margins.

Carbon fibres are relatively expensive, however, wood is cheap and cancover the area of the blade incurring very low costs. Wood itself,however, produces thick inefficient skins in highly stressed blades.Carbon fibres combined with wood may produce thinner skins, which arestructurally efficient and satisfying, Also, wood is highly defecttolerant. The percentage of the total cross-sectional area of the shellcomprised of fibrous composite material is preferably within the rangeof 3% to 30% in the part of the blade having highest content of thefibrous material, more preferred within the range of 6% to 20%.

Likewise, the of the total cross-sectional area of the shell comprisedof fibres is preferably within the range of 2% to 20%, more preferredwithin the range of 4% to 15%.

In a particularly preferred embodiment of the present invention, atleast some of the strips are constituted by hollow tubes formed from afibrous composite material. Thereby, material and weight is saved whileadvantageous structural properties are preserved.

At least some of the strips of the fibrous composite material arepreferably pultrusions, i.e. strips made by pultruding mixture of fibresand a matrix material that is cured after pultrusion, such as aprocessable resin, e.g. vinyl ester. Thereby, a strip having straightfibres and a low void content is obtained. Also, a low content of resinmay be obtained leading to little shrinkage and rapid curing.

It is thus advantageous that the pultrusions have a pultrusion directionsubstantially aligned with a longitudinal direction of the blade inwhich direction the properties of the fibres are required. However,pultrusion terminating joints are stress raisers, so particularattention is being given to these in structural element testing.

The fibrous composite material comprises advantageously a fibre volumefraction of 50% to 90%, preferably from 60% to 80%. In particular, thefibrous composite material may comprise a carbon fibre volume fractionof 50% to 90%, preferably from 60% to 80%.

According to a preferred embodiment at least some of the pre-fabricatedstrips are made from a wooden material as wooden materials are low incosts an light weight, and the material properties of the woodenmaterial may be completed to form the required blade material propertiesby combining with strips of other material types, such as fibrouscomposite materials. The wooden material may be strips of wood, which ifnecessary are glued together in the longitudinal direction of the blade.

A preferred embodiment employs plywood, in particular unidirectionalplywood as the wooden material because of the homogeneous materialproperties. Another type of wooden material that may be employed iscomprised by wooden fibres held in a cured resin. Wood is seeing samedirect stresses, so it is possible to use new joint patterns and gluesusing established design allowables, and still being confident of thestructure of the wooden material.

The layer is, according to one embodiment, at least partly constitutedby strips of a wooden material and strips of a fibrous compositematerial in a sequence along the outer periphery. The sequence maypreferably be an alternating sequence of strips of a wooden material andstrips of a fibrous composite material. The alternating sequencepreferably ranges over only a part of the complete periphery of theblade.

It is advantageous that the layer discussed is part of a sandwichconstruction as discussed previously, i.e. is enclosed in an outer shelland an inner shell made from a fibrous composite material such as glassfibre web held by a cured synthetic resin.

Types of Specimens:

Minibeams—1-beam, 2.5 m long by 150 mm by 150 mm (25 mm thick flanges)with half scale skins. Includes pultrusion terminations, defects, woodjoints. 6 m×1.2 m Aerofoil—Type A designed to fall in direct overstress,testing skins, leading and trailing edge joints. Type B specimen withrelatively thin skins for buckling investigations. 31 m Blade—A bladebuilt in the A131 mould with the same root fixings as the AL40 (72×M30fixings), with skins built with a similar distribution of wood andcarbon as AL40, double webs and similar leading edge joint.

Structural Element Testing Element Test Proving Minibeams 3 pointbending Strength of skins, joints in wood static and pultrusionterminations 6 m aerofoil 4 point bending Leading edge joint, webs and‘A’ Thick skins static joints in skin. 6 m aerofoil ‘B’ 4 point bendingBuckling theory with curved Thin skins static skins 31 m BladeCantilever bending Stiffness, frequency, damping, static edgewise (loadto 1.35 max strain as AL40, distribution as A131). Cantilever bending Asedgewise above but 1.5 max static flatwise strain as AL40, distributionas A131. Stud ring bending (strain gauged) Cantilever bendingAccelerated fatigue regime. Tar- fatigue flatwise getted at 1 millioncycles to sim- ulate A140 lifetime strain cycling. Static flatwise toFailure mode and limits failure Root fixing static pull Confirmation ofroot fixing out and fatigue strength margins 40 m Blade Test Cantileverbending static Stiffness, frequency, damping, proof load to edgewise1.35 extreme. Cantilever bending static As edgewise above proof load to1.35 extreme. flatwise Stud ring bending (strain gauged) Cantileverbending Fatigue regime. Targeted at 5 million cycles fatigue flatwiseequivalent of life with 1.35 load factor. Cantilever bending Fatigueregime. Targeted at 5 million cycles fatigue edgewise equivalent of lifewith 1.35 load factor. Static flatwise to failure Failure mode andlimits Coupon Testing Material Test Proving Carbon PultrusionTension/compression Carbon margins very static & fatigue CRAG high testWood Tension/compression Wood joints perform as static & fatigue AL typewell or better than pre- specimen vious joint types Carbon with woodStatic compression Std Carbon works as pre- wood test dicted with woodin low- est strength compressive stress

The invention may incorporate a lightning protection system comprisingtwo possibly replaceable lightning attractors, preferably close to thetip. One of the lightning attractors are placed on the windward side,and the other lightning attractor is placed on the leeward side. Bothare connected to a width of aluminium mesh or similar material extendingover the fibre reinforced area under the surface layer of gel coat ofthe blade, and are passed down to the root of the blade, where it isearthed.

A radio frequency, e.g. a radar signal, absorption medium may optionallybe infused with the rest of the structure. It is also possible to embedoptical fibres in the blade, either additional to the reinforcing fibresor as a substitute to the reinforcing fibres. Optical fibres may be usedto measure loads on and within the surface of the blade during operationof the wind turbine.

Alternatively, resistance measurement of carbon fibres may be used tomeasure loads on or within the surface of the blade. Also, the carbonfibres used for measuring such loads may be one or more of thereinforcing fibres or may be carbon fibres additional to the reinforcingfibres and dedicated to measuring these loads.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is shown in theaccompanying drawing, of which

FIG. 1 is a cross-section of a blade having a layer comprised of stripsof plywood in alternating sequence with strips of a fibre pultrusion,

FIG. 2 a is a cross-section of a blade similar to the blade of FIG. 1having a different distribution along the periphery of the parts withpultrusion strips,

FIG. 2 b shows a plane view of a blade similar to the blade shown incross-section in FIG. 2 a, thus having a similar distribution along theperiphery of the parts with pultrusion strips,

FIG. 2 c is a photograph of the surface of the blade of FIG. 2 a withthe outer shell of composite material removed, and

FIG. 3 illustrates the vacuum resin infusion process.

DETAILED DESCRIPTION OF THE INVENTION

The blade shown in cross-section in FIG. 1 has a layer comprised of40×40 millimetre strips of birch plywood 1 in alternating sequence with6×40 millimetre strips of a carbon fibre pultrusion 2. The layer 1,2stretches along the central part of the blade between two C-beams 3,4 ofglass fibre web and synthetic resin composite denoted the LE (leadingedge) Web 3 and the TE (travelling edge) Web 4 and replacing the centralinner beam discussed previously. The layer 1,2 is sandwiched between aninner layer and an outer layer 6 of glass epoxy skins that carry shearstress and promote the transverse stiffness of the blade. The spacedefined between the upper and lower shell thus constituted by the birchplywood 1 and the carbon fibre pultrusion 2, and the LE Web 3 and the TEWeb 4 is filled with a balsa wood core 7.

The blade shown in FIGS. 2 a, 2 b and 2 c are similar to the one shownin FIG. 1 with the exception that the reinforcement of carbon fibrepultrusions 2 are situated near the areas of contact between the upperand the lower shell and the LE Web 3 and the TE Web 4, where the stressconcentration is highest. In the embodiment shown, double web is usedinstead of a single web. This is to give sufficient buckling margin onthe skins during compression. Also, the leading web reduces the leadingedge joint shear load, allowing a smaller leading edge joint area. Thisis advantageous during manufacturing of the blade.

The technology is advantageous in that the addition of fibre pultrusionsto a wood construction promotes the stiffness of the construction. Thecarbon fibre pultrusions are not used all along the blade length butonly in the middle 70% where required by the stresses. The blade skincross section may be up to 10% by area of carbon fibre pultrusion In themore highly stressed regions, dispersed throughout the wood composite inthe shown embodiment. The skins are typically 60% of the thickness ofthe blade skins comprised purely by wood, which reduces weight andimproves the structural efficiency in the critical edgewise bendingmode. The outer and inner glass epoxy skins are manufactured with glassfibres oriented plus and minus 45 degrees to the longitudinal directionof the blade.

Pultrusions have the advantage of guaranteeing straight fibres and lowvoid content in the carbon fibre composite itself. Furthermore,pultrusions have the advantage of speeding the blade infusion process asthe fine carbon fibres would otherwise need significantly more time toinfuse. The pultrusion has a high fibre volume fraction, about 70%, witha medium strength but highly processable resin, as example vinyl ester.Preferably, when manufacturing the blade, the resin is supplied with“peelply” on the two long sides, which is removed to produce a cleantextured surface ensuring a good bond.

The manufacturing process of a shell of a blade shown in FIG. 3comprises the steps of applying a gel coat (not shown) to a mould 8followed by a transfer media 9 such as a transfer mesh, 45 degrees glassfibre web 10 and epoxy (not shown) to the mould to create the outerglass epoxy skin. Hereafter the wood and pultrusion strips 1,2 arepositioned and a metal mesh 11 such as an aluminium mesh for thelightning protection is then applied. The shell is then wrapped in acontainer, in the process shown a vacuum bag 12, which is evacuated byexterior means 13. Then, resin is injected from a resin reservoir 14through resin channels 15 formed between adjacent strips, from which theresin spreads throughout the construction driven by the vacuum. Ageneral resin used for Infusion is Prime 20 from SP Systems. Aftercuring of the resin, an inner glass epoxy skin 16 is manufactured on topof the wood and pultrusion strips 1,2.

1. A blade for a wind turbine, wherein at least a third of the totallength, measured from tip to hub, of said blade comprises a layer alongan outer periphery of the cross-section of said blade, wherein the layeris at least partly constituted by a number of pre-fabricated stripsarranged in a sequence along the outer periphery, the pre-fabricatedstrips being arranged side-by-side to form connections between adjacentstrips said connections are oriented substantially orthogonal to thesurface of the layer in a plane orthogonal to the length of the blade,wherein at least one of the pre-fabricated strips is formed from afibrous composite material.
 2. A blade according to claim 1, wherein thestrips of the outer layer are joined by means of resin infusion.
 3. Ablade according to claim 2, wherein the strips of the outer layer arejoined by means of vacuum infusion of a resin.
 4. A blade according toclaim 1, wherein at least some of the pre-fabricated strips areconstituted by hollow tubes formed from a fibrous composite material. 5.A blade according to claim 4, wherein at least some of thepre-fabricated strips of the fibrous composite material are pultrusions.6. A blade according to claim 1, wherein the fibrous composite materialcomprises a fibre volume fraction from 50% to 90%.
 7. A blade accordingto claim 6, wherein the fibrous composite material comprises a fibrevolume fraction of from 60% to 80%.
 8. A blade according to claim 1,wherein the fibrous composite material comprises a carbon fibre volumefraction from 50% to 90%.
 9. A blade according to claim 8, wherein thefibrous composite material comprises a fibre volume fraction of from 60%to 80%.
 10. A blade for a wind turbine, wherein at least a third of thetotal length, measured from tip to hub, of said blade comprises a layeralong an outer periphery of the cross-section of said blade, wherein thelayer is at least partly constituted by a number of pre-fabricatedstrips arranged in a sequence along the outer periphery, at least someof the strips having elongated strip cross-sections in a planeorthogonal to the length of the blade, at least some of these stripsbeing arranged with a short side of the strip cross-section along theouter periphery of the cross-section of the blade, and at least some ofthese strips being arranged with a long side of the strip cross-sectionsubstantially orthogonal to the outer periphery of the cross-section ofthe blade.
 11. A blade for a wind turbine, wherein at least a third ofthe total length, measured from tip to hub, of said blade comprises alayer along an outer periphery of the cross-section of said blade,wherein the layer is at least partly constituted by a number of stripsarranged in a sequence along the outer periphery the strips beingarranged to side-by-side to form connections between adjacent strips,said connections are oriented substantially orthogonal to the surface ofthe layer in a plane orthogonal to the length of the blade.
 12. A bladeaccording to claim 11, wherein the strips are pultruded, the pultrudedstrips have a pultrusion direction substantially aligned with alongitudinal direction of the blade.
 13. A blade according to claim 11,wherein the layer is at least partly constituted by a number of stripsmade from a wooden material arranged in a sequence along the outerperiphery.
 14. A blade according to claim 13, wherein the woodenmaterial is plywood.
 15. A blade according to claim 13, wherein thewooden material is comprised by wooden fibres held in a cured resin. 16.A blade according to claim 13, wherein the layer is at least partlyconstituted by strips of a wooden material and strips of a fibrouscomposite material in a sequence along the outer periphery.
 17. A bladeaccording to claim 16, wherein said sequence is an alternating sequenceof strips of a wooden material and strips of a fibrous compositematerial.
 18. A blade according to claim 11, wherein said layer isenclosed in an outer shell and an inner shell made from a fibrouscomposite material.
 19. A blade according to claim 11, wherein loadmeasuring fibres are enclosed in at least one of an outer shell and aninner shell.
 20. A blade according to claim 19, wherein the loadmeasuring fibres are optical fibres being additional to reinforcingfibres.
 21. A blade according to claim 19, wherein the load measuringfibres are carbon fibres being additional to reinforcing fibres.
 22. Ablade according to claim 19, wherein the load measuring fibres areoptical fibres.
 23. A blade according to claim 19, wherein the loadmeasuring fibres are carbon fibers.
 24. A blade according to claim 11,further comprising lightning protection means, the lightning protectionmeans including lightning attractors incorporated into either one orboth of the shells.
 25. A blade according to claim 24, wherein thelightning attractors are connected to a width of metal mesh or similarmaterial extending over the fibre reinforced area of the shells.
 26. Ablade according to claim 11, further comprising a radio frequencyabsorption medium, the radio frequency absorption medium is incorporatedinto either one or both of the shells.
 27. A blade according to claim11, wherein the pultruded strips are pre-fabricated.
 28. A bladeaccording to claim 11, wherein the pultruded strips are of a fibrouscomposite material.
 29. A method for manufacturing a pre-fabricatedstrip for a blade, the blade comprising a layer material arranged alongan outer periphery of a cross-section of the blade, the methodcomprising: assembling at least two individual materials to constitutethe pre-fabricated strip; selecting at least one of the at least twoindividual materials among fibrous composite materials; inserting theassembled at least two individual material into a container; evacuatingthe container so as to infuse a curing resin to allow the resin to cure;and taking out from the container the assembled and cured strip which isfabricated.
 30. A method according to claim 29, wherein the container isa bag.
 31. A method for manufacturing a shell for a blade, the shellcomprising a layer material arranged along an outer periphery of across-section of the shell, the layer comprising pre-fabricated strips,the method comprising: applying a surface material to a mold for theblade; assembling at least two individual materials to constitute thepre-fabricated strips; selecting at least one of the at least twoindividual materials among fibrous composite materials; positioning theat least two individual material in the mold of the blade; inserting theapplied individual materials and other materials into a container;evacuating the container so as to infuse a curing resin to allow theresin to cure; and de-molding from the mold the shell thus fabricated.32. A method according to claim 31, wherein the surface material is agel coat.
 33. The method of claim 31, further comprising: applying atleast one of a metal mesh, a glass fibre mesh and a transfer media tothe mold of the blade.